1. Field of the Invention
The present invention relates to a vehicle steering wheel which has a horn switch assembly and an air bag module, the vehicle steering wheel can sufficiently absorb an impact force which acts on the back side of the ring portion of the steering wheel in the forward direction.
2. Description of the Related Art
A horn switch assembly which is arranged in a vehicle steering wheel while supporting an air bag module, is disclosed in Japanese Utility Model Publication No. Hei. 2-143734, Japanese Patent Publication No. Hei. 8-225051 or the like as shown in FIGS. 21 to 23.
In the above vehicle steering wheel W, horn switch mechanisms 200 are disposed on both sides of the air bag module 201 as shown in FIG. 23.
Each of the horn switch mechanisms 200 comprises an elongated movable plate 202 coupled with the air bag module 201, a fixed plate 203 coupled with the vehicle steering wheel body under the movable plate 202 and being in association with the movable plate 202, metal coiled springs 204 for upwardly urging the movable plate 202 apart from the fixed plate 203, and mounting bolts 205 which are screwed to a vehicle steering wheel body while engaging the movable plate 202 to put a distance from the fixed plate 203 to the movable plate 202 at a fixed distance.
Movable contacts 206 are provided at both ends of the movable plate 202, while fixed contacts are provided at both ends of the fixed plate 203 in association with the movable contacts 206, respectively.
The number of the mounting bolts 205 used in each of the horn switch mechanisms 200 is two. Those mounting bolts 205 are located near to both ends of said movable plate 202 and slightly closer to the center of said movable plate 202 than the locations of said movable contacts 206. The coiled springs 204 are respectively placed around the mounting bolts 205 while being coaxial with the mounting bolts 205.
The movable contacts 206 are located at both ends of the movable plate 202 in order to avoid such a state that the air bag module 201 is excessively declined to the depressing side of the air bag module, viz., a rolling of the air bag module 201 downward. To this end, a measure is used to prevent such a situation that after the movable contacts 206 are brought into contact with the fixed contacts 207 when the horn is operated, the depressing side of the air bag module 201 further descends. Let us consider a case where the movable contacts 206 are placed at positions on the movable plate 202, which are greatly apart from the urging means and the mounting bolts 205, not at both ends of the movable plate 202. In this case, after the movable contacts 206 are brought into contact with the fixed contacts 207, with progression of the deformation of the ends of the movable plate 202, the urging means is compressed, and the depressing side of the air bag module 201 will possibly descend (downward rolling).
The mounting bolts 205 and the coiled springs 204 are placed at the positions near to the movable contacts 206 located at the ends of the movable plate 202, in order to prevent an upward rolling of the air bag module 201. If the mounting bolts 205 and the coiled springs 204 are placed at positions that are located closer to the center of the movable and fixed plates 202, 203 and apart from the movable contacts 206, the side of the air bag module 201 opposite to the depressing side thereof is excessively inclined upward, viz., an upward rolling of the air bag module 201 tends to occur.
Further, the movable plate 202 includes a substantially rectangular base member 208 disposed above the fixed plate 203 and a mounting plate 209 extending upward from the edge of the central part of the base member 208 and being coupled with and supporting the air bag module 201 by connection bolts 210 to be screwed into the side surfaces of the air bag module 201. The movable plate 202 includes engaging holes 211 provided near to both ends of the base member 208.
The movable plate 202 is upward urged apart from the fixed plate 203 and the heads of two mounting bolts 205 to be screwed into the vehicle steering wheel body are brought into engagement with engaging holes 211 of the movable plate 202, whereby a distance from the movable plate 202 to the fixed plate 203 is put at a fixed distance.
Temporary fastening holes 214 are provided near to the center of the base member 208 of the movable plate 202 in order to temporarily fasten the air bag module 201 before the air bag module 201 is coupled with the mounting plate 209 by utilization of the connection bolts 210. To fasten, the engaging leg portions 213 extended from the air bag module 201 are inserted into the temporary fastening holes 214. In this case, the number of temporarily fastening holes 214 is two since two engaging leg portions 213 extended from the air bag module 201 are provided on each side of the air bag module 201. The engaging leg portions 213 are located closer to the center of the base member 208 than the engaging holes 211 so as not to interfere with the mounting bolts 205.
However, in the aforementioned conventional horn switch mechanisms, the two mounting bolts are screwed into the vehicle steering wheel body to which the fixed plate is fixed. The heads of those mounting bolts are brought into engagement with the edges of the engaging holes of the movable plate. Therefore, those mounting bolts must be electrically insulated from the base member of the movable plate. The insulating spacers 215, 216 are placed around the edges of the engaging holes, respectively. Two insulating spacers and two coiled springs corresponding in number to the number of the mounting bolts are used. This leads to an increase of the number of required component parts.
Actually, each of the two insulating spacers has two members, which are put on the upper and lower sides of each engaging hole. A total of four members are used for the insulating spacers.
A possible measure to reduce the number of the component parts is to use a single plate spring in place of the two coiled springs. However, the necessity of preventing a rolling of the air bag module when the horn is depressed makes it difficult to substitute one plate spring for the two coiled springs.
In the horn switch mechanisms, two holes for temporarily fastening the air bag module, provided in the base member of the movable plate, are located close to the center of the base member. For this reason, a temporary fastening strength of the air bag module is weak.
To fasten the air bag module by means of the connection bolts, the connection bolt of one of the horn switch mechanisms, disposed on both sides of the air bag module, is screwed into the corresponding threaded hole of the air bag module. Then, the connection bolt of the other horn switch mechanism is screwed into the corresponding threaded hole of the air bag module. The air bag module is easy to turn by a torque developed when the bolt is tighten into one of the threaded holes. The mounting plate to which the connection bolts are attached is disposed close to the center of the base member of the movable plate, and the temporary fastening holes for the air bag module are also formed close to the center of the base member. Therefore, the center (position of the connection bolt) of the torque of the air bag module is near to the temporary fastening hole. Therefore, the temporary fastening strength of the air bag module is weak.
A possible measure to increase the temporary fastening strength is to increase the length of the base member of the movable plate and to form the temporary fastening holes at locations near both ends of the elongated base member which correspond to the engaging leg portions of the air bag module. In this case, the movable contacts are located at positions greatly apart from and within both ends of the movable plate. When the horn is depressed, the ends of the movable plate are deformed after the movable contacts are brought into contact with the fixed contacts. The result is that the depressing side of the air bag module will possibly descend (downward rolling). For this reason, the measure is impracticable.
On the other hand, a vehicle steering wheel with an air bag module which can absorb an impact force which acts on the back side of the ring portion is disclosed in Japanese Patent Publication No. Hei. 4-278861.
The vehicle steering wheel W of the above publication is constructed as shown in FIG. 24 such that the back-spoke arm sections 217 are notched at a portion 218, and when an impact acts on the back-spoke arm sections, the back arm sections 217 are plastically deformed.
In this steering wheel, deforming means that is formed in the back-spoke arm section 217 is only one notch.
With this structure, when the notched back-spoke arm section is plastically deformed and comes in contact with the column cover located under the steering wheel, the deformation of the back-spoke arm sections is hindered, to thereby absorb the impact energy insufficiently.
Usually, a lower cover made of synthetic resin is disposed under the vehicle steering wheel. There is a chance that the back-spoke arm section comes in contact with the lower cover, and the impact energy absorption is hindered, even if it is made of synthetic resin.